Ultra structure and functions of primary and secondary xylem
Ultra structure and functions of primary and secondary xylem:-
Xylem Definition:-
> Vascular plants are categorised by their vascular tissues, xylem and phloem, which carry nutrients throughout the plant. In vascular plants, the xylem is a type of tissue that transports water and nutrients from the roots to the leaves. The other form of transport tissue is the phloem, which carries nutrients like sucrose throughout the plant.
> The xylem is a vascular tissue that transports water throughout a plant’s body. The complex processes and various cell types constitute xylem transfer water and dissolved nutrients to maintain and nourish plants.
Primary and Secondary Xylem:- There are two different types of xylem cells based on their origin:
i. Primary Xylem:- Primary development from procambium results in the formation of the primary xylem. It contains protoxylem and metaxylem. Protoxylem grows first, followed by metaxylem and then secondary xylem. Protoxylem lacks tracheids and has narrower vessels than metaxylem.
ii. Secondary Xylem:- During secondary growth, the secondary xylem develops from the vascular cambium. Although secondary xylem is also noticed in the gymnosperm communities Ginkgophyta and Gnetophyta and to a lesser extent in the Cycadophyta members, the two major categories in which secondary xylem can be found are conifers (Coniferae) and angiosperms (Angiospermae).
Structure of Xylem:- Xylem in plants is composed of four different kinds of elements:
i. Tracheids:- Tracheids are tiny conductive elements linked to one another by bordered pits and openings in the secondary cell wall. Tracheids sustain the wood structure in conifers that lack the supporting cells and transport xylem sap. Gymnosperms (conifers) and angiosperms have wood containing a significant amount of tracheids.
ii. Vessels:- The primary conductive cell type in angiosperms is known as a vessel element, which is typically broader in diameter than a tracheid and placed axially, one above the other, to form long tubes known as vessels. Xylem sap is transported by interconduit pits, which permit the lateral flow of solutes across neighbouring conductive elements and the axial transport in tracheary elements. Pits can also connect conduits to the nearby xylem parenchyma cells, which are non-tracheary elements.
iii. Xylem Parenchyma:- Xylem parenchyma cells, which can be positioned either axially or radially, are the last type of wood cells. Although these cells usually have secondary cell walls that are relatively thin and commonly lignified, they conduct various vital functions that are essential for wood and trees.
Xylem Fibre: Xylem fibres are dead cell with a central lumen and lignified walls and provides mechanical support in water transportation.
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Characteristics of Xylem:-
> Xylem, one of the conducting tissues, is responsible for transferring nutrients and water from roots to the stem and leaves of the plant.
> It is composed of specialised cells called tracheary components that carry water.
> Tracheids are the first tracheary component discovered in the xylem.
> Some gymnosperms and other seedless plants only have tracheids as their main component for conducting water.
> Vessel members are the second tracheary component in the xylem. Compared to the tracheids, they are highly specialised cells.
> The main component, also known as the vessel element, transports water in angiosperms even when tracheids are present. It is absent in gymnosperms.
> In addition to the tracheary components, the xylem also has parenchyma tissue and fibre cells.
> The lignified fibre cells give the plants structural support. On the other hand, parenchyma consists of unspecialised, thin-walled cells that are used for storage.
Functions of Xylem:-
> Transporting water and some soluble nutrients, such as minerals and inorganic ions, from the roots to the entire plant is the primary job of the xylem. Long tubes made of xylem cells carry nutrients, and the fluid that passes through the xylem cells is known as xylem sap. Passive transport is used to move these compounds; therefore, no energy is needed.
> Capillary action is the process that enables xylem sap to move upward against gravity. Surface tension causes the liquid to rise in this condition. Water is also facilitated in moving upward through the xylem by strict adherence to the xylem cells. However, it gets tough to work against gravity to carry components as a plant gets taller, so xylem sets a maximum limit on the growth of tall trees.
> Three phenomena cause xylem sap to flow:
i. Root Pressure:- If the water potential of root cells is more negative than the soil’s, water can move by osmosis from the soil into the root, typically due to high solute concentrations. As a result, positive pressure forces the sap up the xylem and toward the leaves. Before the stomata open and enable transpiration to start, root pressure is maximum in the morning. Even in a similar habitat, different plant species can have varying root pressures; examples include up to 145 kPa in Vitis riparia but zero in Celastrus orbiculatus.
ii. Pressure Flow Hypothesis:- The phloem system maintains the sugars made in the leaves and other green tissues, whereas the xylem system carries much lighter solutes, water and minerals. Phloem pressure, much greater than air pressure, can increase to several MPa. This high solute content in the phloem permits xylem fluid to be drawn higher by negative pressure due to the selective interconnection between both systems.
iii. Transpirational Pull:- A similar negative pressure is produced at the apex of a plant by water evaporating from the mesophyll cell surfaces and entering the atmosphere. Surface tension, as a result, creates a negative pressure or stress in the xylem that draws water away from the soil and roots. Hence, the mesophyll cell wall experiences the formation of millions of tiny menisci.